Some customers ask us why they can't choose a 'preview frame' for their videos. This is when you (in the Instagram app) scroll to the frame that you want people to see before clicking on the play button to watch the video in their feed.
Unfortunately at this time we can't support choosing the frame for technical reasons.
Because of how choosing these cover images works, it would require highly precise "swiping" calculations on our side, and we simply wouldn't be able to do it in an accurate and reliable manner. Until something changes like that, we just use the default - which we believe currently is the first frame for most videos. So if the first frame is black/blank, the thumbnail will be black/blank.
We are working on an option which will add a random sideways swipe (so that it won't be the first frame) but we won't be able to guarantee a specific frame at this time.